| Category | Rivers |
| Continent | Karterus |
| Region | Maenedhel, Urizigar |
| Map | Galsour River |
Galsour is a steaming hot river fed from even deeper Underdark regions than Maenedhel and Urizigar. The head of the river is in Berolaz, northeast of the ancient demon ruin Barhaz Halarax. The springs and rivulets serving as the river's head come from a vast cavern of geysers, jets and clouds of steam, and puddles and ponds of boiling water. The source of all this hot water comes from areas of Katerus's Tier 2 Underdark.
Starting from Berolaz, the river travels a southward course and along the way to its end at lake Hatosvir it passes through other geothermal "feeder" caverns of similar make-up; high heat, steam and jets of boiling hot water and pools overflowing forming rivulets that feed into the main river.
All of these areas like the one where the river Galsour starts, are a product of water forced upward under pressure and heat from deeper regions of Katerus's continental landmass.
- Boka'tud, of the Marching Twenty, from his exploration journals Delving Tier 2 Karterus - "Source of the Galsour"
Galsour's water temperature cools as it nears and then merges with lake Hatosvir. Outside of dry spells the waters of this lake are always overflowing into natural channels diving downward and serving as links between Huzar-Arân and the sector below it. There are hundreds of these channels some so small to be barely noticeable and others large enough to serve as passages for Underdark travelers. Galsour's overflowing waters go down these channels back to where it all started in super-heated regions below, where the whole process begins anew. The structure behind this movement of water between a Tier 2 and Tier 1 sector of the Underdark is called the Kossuth Aqueduct.